Re: [Qemu-devel] [PATCH] migration: introduce decompress-error-check

[Peter Xu]

On Fri, Apr 27, 2018 at 06:40:09PM +0800, Xiao Guangrong wrote:
> 
> 
> On 04/27/2018 05:31 PM, Peter Xu wrote:
> > On Fri, Apr 27, 2018 at 11:15:37AM +0800, Xiao Guangrong wrote:
> > > 
> > > 
> > > On 04/26/2018 10:01 PM, Eric Blake wrote:
> > > > On 04/26/2018 04:15 AM, address@hidden wrote:
> > > > > From: Xiao Guangrong <address@hidden>
> > > > > 
> > > > > QEMU 2.13 enables strict check for compression & decompression to
> > > > > make the migration more robuster, that depends on the source to fix
> > > > 
> > > > s/robuster/robust/
> > > > 
> > > 
> > > Will fix, thank you for pointing it out.
> > > 
> > > > > the internal design which triggers the unexpected error conditions
> > > > 
> > > > 2.13 hasn't been released yet.  Why do we need a knob to explicitly turn
> > > > off strict checking?  Can we not instead make 2.13 automatically smart
> > > > enough to tell if the incoming stream is coming from an older qemu
> > > > (which might fail if the strict checks are enabled) vs. a newer qemu
> > > > (the sender gave us what we need to ensure the strict checks are
> > > > worthwhile)?
> > > > 
> > > 
> > > Really smart.
> > > 
> > > How about introduce a new command, MIG_CMD_DECOMPRESS_ERR_CHECK,
> > > the destination will do strict check if got this command (i.e, new
> > > QEMU is running on the source), otherwise, turn the check off.
> > 
> > Why not we just introduce a compat bit for that?  I mean something
> > like: 15c3850325 ("migration: move skip_section_footers",
> > 2017-06-28).  Then we turn that check bit off for <=2.12.
> > 
> > Would that work?
> 
> I am afraid it can not. :(
> 
> The compat bit only impacts local behavior, however, in this case, we
> need the source QEMU to tell the destination if it supports strict
> error check.

My understanding is that the new compat bit will only take effect when
at destination.

I'm not sure I'm thinking that correctly. I'll give some examples.

When we migrate from <2.12 to 2.13, on 2.13 QEMU we'll possibly with
(using q35 as example, always) "-M pc-q35-2.12" to make the migration
work, so this will let the destination QEMU stop checking
decompressing errors.  IMHO that's what we want so it's fine (forward
migration).

When we migrate from 2.13 to <2.12, on 2.12 it'll always skip checking
decompression errors, so it's fine too even if we don't send some
compress-errored pages.

Then, would this mean that the compat bit could work too just like
this patch?  AFAIU the compat bit idea is very similar to current
patch, however we don't really need a new parameter to make things
complicated, we just let old QEMUs behave differently and
automatically, then user won't need to worry about manually specify
that parameter.

Thanks,

-- 
Peter Xu